Net Submarine Groundwater‐Derived Dissolved Inorganic Nutrients and Carbon Input to the Oligotrophic Stratified Karstic Estuary of the Krka River (Adriatic Sea, Croatia)

Abstract

Submarine groundwater discharge (SGD) is a significant source of biogenic elements in estuaries, and relevant studies in karstic estuaries are scarce. Krka River Estuary (KRE), located on the eastern Adriatic Sea (Croatia), is a typical oligotrophic stratified karstic estuary. In this study, based on 226Ra and 228Ra, the total SGD flux into the KRE surface layer was estimated to be (12.8–16.2) × 105 m3/day. A conservative estimation of the fresh groundwater flux was (5.0–8.3) × 105 m3/day, which accounts for 10–17% of the Krka River discharge into the estuary. By establishing water and nutrient budgets in the KRE surface layer, we found that SGD dominated the nutrient sources, although it accounted for a small portion of the total inflow water. Specifically, net SGD‐derived dissolved inorganic nitrogen (DIN) and silicates contributed 58–90% and 24–64%, respectively, to the total input fluxes. These results indicate that SGD was a major external nutrient source, in which net SGD‐derived high DIN flux and high DIN to dissolved inorganic phosphorus ratio may affect productivity in the KRE ecosystem and nearby Adriatic Sea. Additionally, net SGD‐derived dissolved inorganic carbon (DIC) flux in the KRE (1.53 mol · m2 · day) was much higher than those in most estuaries worldwide, suggesting that the DIC‐enriched karst aquifers are important sources for global carbon cycle. Therefore, the impact of net SGD‐derived DIC from karst aquifers on coastal seas will likely become more evident and substantial with further development of global climate change, such as sea level rise.

Tracing Nutrients and Organic Matter Changes in Eutrophic Wenchang (China) and Oligotrophic Krka (Croatia) Estuaries: A Comparative Study

“…Accordingly, the Chla data and diagnostic pigments, for both estuaries, were cited from Vrana Špoljarić et al (2021). Also note that some data of the KRE (for DIN; Liu et al, 2019b) and the WRE (for nutrients and POC; Vrana Špoljarić et al, 2021) are published in previous literature, with different foci compared with the current study.…”

“…The mean runoff varies from 40 to 60 m 3 /s (Bonacci et al, 2006). During our field survey, the Krka River flow was 60% of the annual mean (Liu et al, 2019b). Tide effect is very limited, with tidal height < 0.5 m, which results in permanent stratification (Legović et al, 1994).…”

“…Tide effect is very limited, with tidal height < 0.5 m, which results in permanent stratification (Legović et al, 1994). The fresh water above the brackish layer has a residence time of 2 to 3 days (Liu et al, 2019b), shorter than that for WRE (1 week). With rocky riverbank, water in the KRE is much cleaner (low in TSM) relative to the turbid WRE.…”

“…Stratification throughout the year is the key feature (Zutić and Legović, 1987), in which the permanent halocline has been recognized as a layer of highest microbiological activity and accumulation of OM (Zutić and Legović, 1987;Fuks et al, 1991). As a karstic estuary, the nutrients and carbon in the KRE have a strong sub-ground contribution (Liu et al, 2019b). POC concentration in the KRE is very low (Vrana Špoljarić et al, 2021), largely due to the limited in situ primary production (Cauwet, 1991), except for the summer season (Cindrić et al, 2015).…”

“…However, there are some anthropogenic activities in the lower reaches of the KRE, including urbanization, aquaculture (mussels farming), and shipment (Padan et al, 2020). Despite a moderate anthropogenic influence, nutrients and dissolved organic carbon (DOC) concentrations in the KRE remain at very low levels for the past decades (Cauwet, 1991;Sempéré and Cauwet, 1995;Cetinić et al, 2006;Cindrić et al, 2015;Liu et al, 2019b).…”